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Technical Brief

Comparisons of Anterior Plate Screw Pullout Strength Between Polyurethane Foams and Thoracolumbar Cadaveric Vertebrae

[+] Author and Article Information
Srinidhi Nagaraja

Office of Science and Engineering Laboratories,
Division of Applied Mechanics,
Center for Devices and Radiological Health,
U.S. Food and Drug Administration,
10903 New Hampshire Avenue,
Building 62, Room 2210,
Silver Spring, MD 20993-0002

Vivek Palepu

Office of Science and Engineering Laboratories,
Division of Applied Mechanics,
Center for Devices and Radiological Health,
U.S. Food and Drug Administration,
Silver Spring, MD 20993

1Corresponding author.

Manuscript received May 24, 2016; final manuscript received August 5, 2016; published online August 24, 2016. Assoc. Editor: Brian D. Stemper.

J Biomech Eng 138(10), 104505 (Aug 24, 2016) (6 pages) Paper No: BIO-16-1219; doi: 10.1115/1.4034427 History: Received May 24, 2016; Revised August 05, 2016

Synthetic polyurethane foams are frequently used in biomechanical testing of spinal medical devices. However, it is unclear what types of foam are most representative of human vertebral trabecular bone behavior, particularly for testing the bone–implant interface. Therefore, a study was conducted to compare polyurethane foam microstructure and screw pullout properties to human vertebrae. Cadaveric thoracolumbar vertebrae underwent microcomputed tomography to assess trabecular bone microstructure. Spine plate screws were implanted into the vertebral body and pullout testing was performed. The same procedure was followed for eight different densities (grades 5–30) of commercially available closed cell (CCF) and open cell foams (OCF). The results indicated that foam microstructural parameters such as volume fraction, strut thickness, strut spacing, and material density rarely matched that of trabecular bone. However, certain foams provided mechanical properties that were comparable to the cadavers tested. Pullout force and work to pullout for screws implanted into CCF grade 5 were similar to osteoporotic female cadavers. In addition, screw pullout forces and work to pullout in CCF grade 8, grade 10, and OCF grade 30 were similar to osteopenic male cadavers. All other OCF and CCF foams possessed pullout properties that were either significantly lower or higher than the cadavers tested. This study elucidated the types and densities of polyurethane foams that can represent screw pullout strength in human vertebral bone. Synthetic bone surrogates used for biomechanical testing should be selected based on bone quantity and quality of patients who may undergo device implantation.

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References

Figures

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Fig. 1

Representative micro-CT images of different densities of closed cell foams (CCF), open cell foams (OCF), and human vertebral trabecular bone

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Fig. 2

Screw pullout testing setup of closed cell foam (left), open cell foam (center), and cadaveric vertabra (right)

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Fig. 3

(a) Volume fraction (mm3/mm3) of different grades of open cell foams, closed cell foams, and cadaveric trabecular bone. F symbol indicates that foam grade is similar (p ≥ 0.50) to female cadaveric specimens. M symbol indicates foam grade is similar (p ≥ 0.36) to male cadaver specimens, (b) strut thickness (mm) of different grades of foam and cadaveric specimens. All grades of foam were significantly different from each other (p < 0.002).

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Fig. 4

(a) Strut spacing (mm) of different grades of foam and cadaveric specimens and (b) material density (mg HA/cc) of different grades of foam and cadaveric specimens. All grades of foam had significantly different density and strut spacing compared to each other (p < 0.001).

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Fig. 5

(a) Peak pullout force (PPF) for cadaveric specimens and different grades of foam (b) pullout stiffness for cadaveric specimens and different grades of foam (c) work to PPF for cadaveric specimens and different grades of foam and (d) work after PPF for cadaveric specimens and different grades of foam. F symbol indicates that foam grade was similar (p ≥ 0.66) to female cadaveric specimens. M symbol indicates that foam grade was similar (p ≥ 0.15) to male cadaveric specimens.

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